Allotypes of immunoglobulin kappa chains in the laboratory rat, controlled by the RI-1 locus, have been shown to differ by 15% of their C-region amino acid sequence. This finding has raised many questions regarding the genetic structure and evolution of immunoglobulin. Recent studies have shown the RI-lb (LEW) serotype to be antigenically complex, and its multiple specificities are widely distributed, in different combinations, among Asian and Australian rodents. The objectives of this proposal are to examine the chemical, serological, and genetic structure of rat kappa chains. Australian Rattus are uniquely suited for such studies, consisting of a closely related group of species and subspecies whose taxonomic relationships are well understood, but which nevertheless show extensive diversity with respect to RI-lb cross-reactivity. The serological structure of their kappa chains will be determined using conventional and hybridoma antibody, and correlated with the amino acid sequences determined by automated techniques. Hybrids and backcrosses will be made between species and subspecies differing in RI-1 reactivity as well as a number of biochemical and chromosomal markers (ranging from 2N equals 32 to 2N equals 50). In this way we hope to determine the chromosomal localization and genetic linkage of the kappa chain locus. In order to elucidate the genetic basis for RI-1 allotypy and the existence of two kappa chains in LOU rats, restriction mapping, DNA cloning and sequencing will be carried out on kappa chain genes from selected laboratory and Australian rats. This will directly test the possibility of RI-1 "alleles" actually having evolved as duplicated genes (latent allotypes), and enable comparison of kappa gene structure between forms showing extensive serological diversity. The results of these studies will have a direct bearing on the mechanisms involved in the evolution and inheritance of immunoglobulins, on the structural basis of Ig antigenicity and immunogenicity, on the possible phylogenetic significance of kappa chain allotypes, and the comparative structure of the mammalian genome.